1. Field of the Invention
The present invention relates to a semiconductor laser, and more particularly to a semiconductor laser comprising a II-VI group compound semiconductor lattice-matched to a substrate of GaAs or GaP, for emitting a blue or ultraviolet radiation.
2. Description of the Prior Art
There has been a growing demand in the art for short-wavelength semiconductor lasers for blue or ultraviolet emission to meet the requirements for recording and reproducing high-density and high-resolution information on optical and magneto-optic disks.
Blue or ultraviolet semiconductor lasers are required to be made of direct-transition-type materials having large bandgaps Eg. In particular, double hetero-junction semiconductor lasers are required to have a cladding layer having a greater bandgap than an active layer thereof.
Semiconductor lasers have a substrate on which semiconductor layers are deposited by way of epitaxial growth. Such a substrate should preferably be a single-crystal substrate of GaAs or GaP that is widely used in various general compound semiconductor devices, has good crystalline properties, can be manufactured with ease, is readily available, and inexpensive.
II-VI group compound semiconductors, particularly IIb-VI group compound semiconductors or mixed semiconductors, are promising as optical device materials because they have a direct-transition-type band structure.
The direct-transition-type IIa-VI group compounds with a large bandgap Eg are also attracting attention for use as fluorescent materials. However, the IIa-VI group compounds are chemically unstable since they are hydrolyzable in air, and their basic properties are still unknown.
Therefore, it is considered better to construct optical devices of IIb-VI group compounds. One problem with using IIb-VI group compounds is that it is difficult to select materials having different bandgaps for active and cladding layers, respectively, even if mixed crystals of IIb-VI group compounds are used.
More specifically, FIG. 1 of the accompanying drawings shows the relationship between lattice constants a and bandgaps Eg of various compound semiconductors. As can be seen from FIG. 1, mixed crystals of IIb-VI group compounds have large bowing parameters, which make it difficult to combine materials that are lattice-matched to each other and have a large bandgap difference.
Semiconductor lasers that are currently proposed for emission in a blue range include a semiconductor laser having an active layer of ZnSe and a cladding layer of a superlattice of ZnSSe and ZnSe, and a semiconductor laser having an active layer of ZnCdS and a cladding layer of ZnSSe. However, the bandgap difference between the active and cladding layers of each of these semiconductor lasers is 100 MeV or less, which poses a problem on the function of the cladding layer, i.e., the function of optical and carrier confinement.
Japanese Laid-Open Patent Publication No. 1-16998 discloses a blue semiconductor laser comprising a substrate of GaAs, an active layer of ZnSe.sub.x S.sub.1-x, and a cladding layer of Zn.sub.x Mg.sub.1-x Te. Japanese Laid-Open Patent Publication No. 63-233576 also shows a p-n junction light-emitting device. However, the presence of Zn.sub.x Mg.sub.1-x Te that lattice-matches GaAs or GaP has not been experimentally confirmed.
Under the above circumstances, semiconductor lasers of a double heterostructure which have a bandgap Eg .ltoreq.2.7 eV have not been in practical use yet, and semiconductor lasers having a substrate of GaAs or GaP which are excitable for continuous oscillation at room temperature have not been provided either.